How to upload arbitrary file contents cross-domain

Update: Since publishing details of this technique it has been used to exploit CRSFable file upload forms on Facebook , Flickr, Imgur, minus.com, Tumblr.com and others. It seems that many file upload forms lack anti CSRF tokens. 

HTML5, together with its sister specifications (XMLHTTPRequest level 2, File API etc.) has a really interesting property when it comes to security. Websites that are coded securely get new tools allowing them to be even more secure. Yet poorly coded websites might be prone to new flavours of attack. It makes good even better, and bad even worse.

The best example of this would be the Cross Origin Resource Sharing (CORS) specification commonly known as Cross Domain AJAX. Back in the days, AJAX request could not be sent cross domain - now, in all current browsers, they can. Does it affect security? Sure it does - even Facebook got hacked with it. While the specification was designed with security in mind, fully opt-in, introducing new headers and preflight mode, there are sites in the Internet that suddenly got vulnerable once surfers upgraded their browsers.

Continuing the fun with file upload issues in current browers, today I'd like to show you how to upload a file:

1. from victim's browser
2. with arbitrary filename
3. with arbitrary content
4. without user interaction
5. .. to another domain.

Filejacking: How to make a file server from your browser (with HTML5 of course)

Back in the days of browser wars, there was a joke: Internet Explorer is the only web browser that makes Internet browse your computer. Through various security flaws, IE was exploitable and allowed for remote code execution that could e.g. steal your sensitive files.
But now the times are different. It's not that easy to exploit current browsers, they get patched (relatively) quickly. Attackers cannot easily access your files using browsers vulnerabilities, so they turn to the weakest link - users. In this post we'll try to explore what current browsers can do with your files.